This invention is an improvement over the state of the art double dome combustor used in gas turbine engines.
Many gas turbine engines incorporate annular combustors to obtain the benefits of compactness, performance, weight, cost and durability. In those combustors of the single annular type where there is only a single annular array of fuel injectors and swirlers in the front section of the combustor, there is a relationship of dome height to combustor length (L/H) which largely governs the length of the combustor. The L/H ratio of modern combustor designs generally falls into the 2.0 to 2.5 range. The L/H ratio affects such characteristics as combustor exit temperature distribution including the circumferential integrated temperature profile and the pattern factor, and this is of particular importance to the stationary inlet turbine vane.
Generally speaking the exit temperature distribution is enhanced with higher L/H combustor ratios because longer lengths enhance mixing, and hence uniformity; however there are opposing consideration of engine weight, cost and durability which argue in favor of low L/H ratios, or shorter combustor designs. Shorter designs require less cooling air by virtue of the reduced surface area to be cooled, and are generally favored for engines which put a premium on high thrust to weight ratios.
It has been demonstrated that a double annular combustor can provide a number of benefits over the single annular combustor. Basically, a double annular combustor, also known as a double dome combustor, comprises two concentric single dome annular combustors. The benefits of the double dome combustor include:
1. Length. The combustors are shorter due to the potential of halving the L/H ratio.
2. Operability. By implementing two annular arrays of fuel injectors which is basic to the double annular design, the fuel may be staged so that the combustor operates with stability over a broader range of fuel/air ratios.
3. Emission Reduction. The short length and fuel staging flexibility allows better control for the reduction of gaseous emissions including hydrocarbons, CO and NOX.
The state of the art of double dome combustor apparatus is represented by a number of U.S. patents:
U.S. Pat. No. 4,903,492 issued to King on Sep. 7, 1988 discloses a dilution air dispensing apparatus for a double dome combustor with dilution air dispensing holes that are staggered on the opposite walls of the centerbody. The wakes on the downstream side of the jets provide avenues of access for the deflected combustion gases to continue their travel across the combustor and into the regions downstream of the opposite domes. The gases then mix with the remaining undeflected gases from the domes and with the spent dilution air before reaching a combustor exit plane. This dilution method is common to essentially all combustors on the inner and outer shells.
U.S. Pat. No. 4,173,118 issued to Kawaguchi on Nov. 6, 1979 discloses a fuel combustion cylinder having a double concentric combustion cylinder which includes a rich mixture zone, a lean mixture zone and a dilution zone.
U.S. Pat. No. 3,306,333 issued to Mock on Feb. 28, 1967; Mock patent is directed to a combustor of the flame tube type and toroidal or smoke ring type of gas circulation in the primary zone.
U.S. Pat. No. 4,237,694 issued to Wood et al on Dec. 9, 1980 describes a combustor for a gas turbine engine which has a central duct partially surrounded by an annular duct. Each duct has an array of swirl vanes at their upstream ends and fuel inlet apertures are located downstream of the respective arrays of swirl vanes.
U.S. Pat. Nos. 3,606,421 issued to Goddard on Aug. 12, 1952, 3,820,324 issued to Grindley et al on Jun. 28, 1974, 3,851,465 issued to Verdouw on Dec. 3, 1974, 4,113,425 issued to von Linde et al on Sep. 12, 1978, and 4,195,475 issued to Verdouw on Apr. 1, 1980 provide additional background information on the state of the field of art.
U.S. Pat. No. 3,720,058 issued to Collinson et al discloses a combustor with three annular sets of injectors.
U.S. Pat. No. 4,194,358 issued to Stenger discloses a double annular combustor having means to separate the gases.
U.S. Pat. No. 4,215,536 issued to Rudolph discloses a mixer apparatus for mixing core and fan streams.
U.S. Pat. No. 4,246,758 issued to Caruel et al discloses a double annular combustor with an internal wall between the combustors.
U.S. Pat. No. 4,305,255 discloses a double annular combustor without an internal wall.
As pointed out in King U.S. Pat. No. 4,903,492, combustor length of advanced engines were being reduced for the purpose of reducing overall engine length and weight. The double dome combustor served as a means for reducing combustor length while meeting other important design criteria; however, in short double dome combustor designs, some amount of centerbody dilution air is needed. This additional centerbody dilution air, along with the dilution air from the combustor inner and outer walls, provides cooling to the center portion of the combustor exit stream, and is also necessary to achieve the desired temperature gradient variation (profile shape) across the combustor exit stream. The circumferential variations in temperature must also be limited; however, this condition is particularly difficult due to the shortness of the combustor and the very limited length which is available for mixing between the locations where dilution air is admitted and the combustor exit.
In accordance with King, some of the described problems were alleviated in the double dome combustor by inclining a pair of dilution jets in a slightly downstream direction from the dome and staggering the dilution holes located on opposite walls of the centerbody to produce what King described as a powerful aerodynamic mixer. The blockage effect created by the inclined jets caused a portion of the approaching combustion gases to accelerate and to turn toward the regions downstream of the adjacent or opposite domes.
The present invention is an improvement over the prior art double dome combustors and over the King apparatus in that it provides a daisy-type mixer downstream of the domes to increase the mixing perimeter of the region between the two combustion areas and to provide complete mixing of the gases before reaching the exit plane of the combustor, at low pressure loss. Daisy mixers are very efficient in accomplishing lateral mixing of streams of unequal temperature at expenditure of low pressure loss. They require less pressure drop than the typical dilution jets, and hence the efficiency of the engine in enhanced.